Dengue fever (DF) is an acute febrile disease caused by one of four closely related virus serotypes (DEN-1, DEN-2, DEN-3, and DEN-4). Dengue fever is classified based on its clinical characteristics into classical dengue fever, or the more severe forms, dengue hemorrhagic fever syndrome (DHF), and dengue shock syndrome (DSS). Recovery from infection from one serotype produces life-long immunity to that particular serotype, but provides only short-lived and limited protection against any of the other serotypes (37). Dengue is a member of the Flaviviridae family which are enveloped, positive-sense RNA viruses whose human pathogens also include West Nile virus (WNV), yellow fever virus (YFV), Japanese encephalitis virus (JEV), and tick-borne encephalitis virus (TBEV) among others. Dengue transmission is via the bite of an infected Aedes aegypti mosquito which is found in tropical and sub-tropical regions around the world.
Each year regional epidemics of dengue cause significant morbidity and mortality, social disruption and substantial economic burden on the societies affected both in terms of hospitalization and mosquito control. Dengue is considered by the World Health Organization (WHO) to be the most important arthropod-borne viral disease with an estimated 50 million cases of dengue infection, including 500,000 DHF cases and 24,000 deaths worldwide each year (37, 38). WHO estimates that forty percent of the world's population (2.5 billion people) are at risk for DF, DHF, and DSS (37). Dengue is also a NIAID Category A pathogen and in terms of bio-defense, represents a significant threat to United States troops overseas. Dengue is an emerging threat to North America with a dramatic increase in severe disease in the past 25 years including major epidemics in Cuba and Venezuela, and outbreaks in Texas and Hawaii (4). Failure to control the mosquito vector and increases in long-distance travel have contributed to the increase and spread of dengue disease. The characteristics of dengue as a viral hemorrhagic fever virus (arthropod-borne, widely spread, and capable of inducing a great amount of cellular damage and eliciting an immune response that can result in severe hemorrhage, shock, and death) makes this virus a unique threat to deployed military personnel around the world as well as to travelers to tropical regions. Preparedness for both biodefense and for the public health challenges posed by dengue will require the development of new vaccines and antiviral therapeutics.
Dengue causes several illnesses with increasing severity being determined in part by prior infection with a different serotype of the virus. Classic dengue fever (DF) begins 3-8 days after the bite of an infected mosquito and is characterized by sudden onset of fever, headache, back pain, joint pain, a measles-like rash, and nausea and vomiting (21). DF is frequently referred to as “breakbone” fever due to these symptoms. The disease usually resolves after two weeks but a prolonged recovery with weakness and depression is common. The more severe form of the disease, dengue hemorrhagic fever (DHF) has a similar onset and early phase of illness as dengue fever. However, shortly after onset the disease is characterized by high fever, enlargement of the liver, and hemorrhagic phenomena such as bleeding from the nose, mouth, and internal organs due to vascular permeability (38). In dengue shock syndrome (DSS) circulatory failure and hypovolaemic shock resulting from plasma leakage occur and can lead to death in 12-24 hours without plasma replacement (38). The case fatality rate of DHF/DSS can be as high as 20% without treatment. DHF has become a leading cause of hospitalization and death among children in many countries with an estimated 500,000 cases requiring hospitalization each year and a case fatality rate of about 5% (37).
The pathogenesis of DHF/DSS is still being studied but is thought to be due in part to an enhancement of virus replication in macrophages by heterotypic antibodies, termed antibody-dependent enhancement (ADE) (8). During a secondary infection, with a different serotype of dengue virus, cross-reactive antibodies that are not neutralizing form virus-antibody complexes that are taken into monocytes and Langerhans cells (dendritic cells) and increase the number of infected cells (7). This leads to the activation of cytotoxic lymphocytes which can result in plasma leakage and the hemorrhagic features characteristic of DHF and DSS (21). This antibody-dependent enhancement of infection is one reason why the development of a successful vaccine has proven to be so difficult. Although less frequent, DHF/DSS can occur after primary infection (33), so virus virulence (16) and immune activation are also believed to contribute to the pathogenesis of the disease (26).
Dengue is endemic in more than 100 countries in Africa, the Americas, the Eastern Mediterranean, Southeast Asia and the Western Pacific. During epidemics, attack rates can be as high as 80-90% of the susceptible population. All four serotypes of the virus are emerging worldwide, increasing the number of cases of the disease as well as the number of explosive outbreaks. In 2002 for example, there were 1,015,420 reported cases of dengue in the Americas alone with 14,374 cases of DHF, which is more than three times the number of dengue cases reported in the Americas in 1995 (24).
The dengue genome, approximately 11 kb in length, consists of a linear, single stranded, infectious, positive sense RNA that is translated as a single long polyprotein (reviewed in (29). The genome is composed of seven nonstructural (NS) protein genes and three structural protein genes which encode the nucleocapsid protein (C), a membrane-associated protein (M), and an envelope protein (E). The nonstructural proteins are involved in viral RNA replication (35), viral assembly, and the inflammatory components of the disease (19). The structural proteins are involved mainly in viral particle formation (22). The precursor polyprotein is cleaved by cellular proteinases to separate the structural proteins (18), while a virus-encoded proteinase cleaves the nonstructural region of the polyprotein (6). The genome is capped and does not have a poly(A) tail at the 3′ end but instead has a stable stem-loop structure necessary for stability and replication of the genomic RNA (3). The virus binds to cellular receptors via the E protein and undergoes receptor-mediated endocytosis followed by low-pH fusion in lysosomes (20). The viral genome is then uncoated and translated into the viral precursor polyprotein. Co- and posttranslational proteolytic processing separates the structural and nonstructural proteins. The RNA-dependent RNA polymerase along with cofactors synthesizes the minus-strand RNA which serves as a template for the synthesis of the progeny plus-strand RNA (25). Viral replication is membrane associated (1, 34). Following replication, the genome is encapsidated, and the immature virus, surrounded by a lipid envelope buds into the lumen (9). The envelope proteins become glycosylated and mature viruses are released outside the cell. Essential stages or process during the virus life cycle would be possible targets for inhibition from an antiviral drug and include binding of the virus to the cell through the E protein, uptake of the virus into the cell, the capping mechanism, the viral proteinase, the viral RNA-dependent RNA polymerase, and the viral helicase.
Current management of dengue virus-related disease relies solely on vector control. There are no approved antivirals or vaccines for the treatment or prevention of dengue. Ribavirin, a guanosine analogue, has been shown to be effective against a range of RNA virus infections and works against dengue in tissue culture by inhibiting the dengue 2′-O-methyltransferase NS5 domain (2, 10). However, ribavirin did not show protection against dengue in a mouse model (15) or a rhesus monkey model (17), instead it induced anemia and thrombocytosis. While there are no currently available approved vaccines, multivalent dengue vaccines have shown some limited potential in humans (5, 12, 13, 28). However, vaccine development is difficult due to the presence of four distinct serotypes of the virus which each cause disease. Vaccine development also faces the challenge of ADE where unequal protection against the different strains of the virus could actually increase the risk of more serious disease. Therefore there is a need for antiviral drugs that target all of the serotypes of dengue. An antiviral drug administered early during dengue infection that inhibits viral replication would prevent the high viral load associated with DHF and be an attractive strategy in the treatment and prevention of disease. An antiviral drug that inhibits viral replication could be administered prior to travel to a dengue endemic region to prevent acquisition of disease, or for those that have previously been exposed to dengue, could prevent infection by another serotype of virus and decrease the chance of life-threatening DHF and DSS. Having an antiviral drug would also aid vaccine development by having a tool at hand to treat complications that may arise due to unequal immune protection against the different serotypes. Although a successful vaccine could be a critical component of an effective biodefense, the typical delay to onset of immunity, potential side-effects, cost, and logistics associated with large-scale civilian vaccinations against a low-threat risk agent suggest that a comprehensive biodefense include a separate rapid-response element. Thus, there remains an urgent need to develop a safe and effective product to protect against flavivirus infection.